The invention relates to an arrangement for coupling out an output current from a load current by a load, particularly a deflection coil of a cathode ray tube by means of an output resistor, which arrangement comprises an output current mirror, in which a control value in the form of the difference between two voltages dropping across two resistors for controlling the load current is generated and in which a reference current bank is provided with a current mirror circuit whose input receives a constant current for generating constant currents.
Such an arrangement is known from IC TDA 4866 marketed by Philips Semiconductors. This IC is used to gain a control value from the current flowing through a deflection coil of a display tube so as to control this current. To this end, an output current is gained via an output resistor. The output current is coupled to two transistors which are cross-coupled to two output transistors of an output current mirror. The cross-coupling of these transistors serves to reduce the temperature dependence of the circuit. Actually, this is not effected completely so that a temperature dependence of the circuit still remains. Furthermore, the cross-coupled output stage has the drawback that it tends to oscillate.
It is an object of the invention to provide an arrangement of the type described in the opening paragraph which has substantially no temperature dependence and no oscillation tendencies.
According to the invention, this object is achieved in that the output current is coupled to the emitter of at least a first output transistor and at least a second output transistor of the output current mirror, in that the reference current bank supplies a first reference current which, together with the output current, is coupled to the emitter of the first output transistor and the second output transistor of the output current mirror, in that the current through the collector of the first output transistor of the output current mirror is coupled to the input of a first current mirror, in that the output current of the first current mirror is coupled to the collector of an input transistor of the output current mirror, in that a second current mirror is provided to whose input the current through the collector of the second output transistor of the output current mirror is coupled and whose output current is coupled to a first resistor, in that a third current mirror is provided to whose input a second reference current from the reference current bank is coupled and whose output current is coupled to a second resistor, in that the input transistor of the output current mirror has the same emitter area as that of the first output transistor, and in that the difference between the voltages dropping across the first resistor and the second resistor represents the control value.
In the arrangement according to the invention, a load current is coupled out by means of an output resistor by a load, for example, the deflection coil of a television display tube, a monitor tube or the winding of a motor. This current which is coupled out is relatively small as compared with the current flowing through the load. It is an object of the arrangement to gain a control value in dependence upon this output current which in its turn is dependent on the load current through the load, which control value is suitable for controlling the load current. It is essential that no additional errors due to, for example, temperature dependencies occur when gaining this control value.
In the arrangement according to the invention, this is achieved by means of the following measures. The output current is not coupled only to an output transistor of the output current mirror but to all output transistors of the output current mirror. The output current mirror has at least a first and at least a second output transistor. In other words, the output current mirror has two categories of output transistors, at least one of which is each time available.
Furthermore, a reference current bank is provided whose input receives a constant current and which comprises a current mirror circuit. The reference current bank supplies a first reference current which, together with the output current, is coupled to the emitter of the first and the second output transistor of the output current mirror. This is an essential difference with the state-of-the-art circuit.
To ensure that the voltage through the base-emitter paths of the input transistor of the output current mirror and the output transistors of the output current mirror is minimal, or tends towards zero, the current through the collector of the first output transistor of the output current mirror is coupled to the input of a first current mirror. The output current of this first current mirror is coupled again to the collector of the input transistor of the output current mirror. It is thereby achieved that a current of the same value as in the output transistors flows in the input transistor of the output current mirror. This in turn has the result that the base-emitter voltage tends towards zero. It is thereby achieved that there is no noticeable temperature dependence of the circuit. A condition for this is that the input transistor(s) of the output current mirror has (have) the same emitter areas as those of the first input transistor(s).
Furthermore, a second current mirror is provided to whose input the current through the second output transistor(s) of the output current mirror is coupled. The output current of this second current mirror is coupled to a first resistor.
The reference current bank also supplies a second reference current which is coupled to a third current mirror whose output current is coupled to a second resistor. In this circuitry, the difference of the voltage dropping across the first and the second resistor supplies the control value. The value of the first reference current in relation to the second reference current is dependent on the number of output transistors of the output current mirror. When the output current mirror has n output transistors with equal emitter areas, the first reference current is n times as large as the second reference current.
As a result, the circuit arrangement has a clearly reduced temperature drift because the current gain of the transistors does not influence the behavior of the circuit due to the base-emitter voltage tending towards zero in the output current mirror. Furthermore, the circuit arrangement has a stable behavior in a control loop, also in the MHz range.
As already explained hereinbefore, one or more output transistors of the first and the second category may be arranged in the output current mirror. Moreover, these output transistors may fundamentally have emitters with different areas so that also different currents flow. However, the simplest solution, which is also efficient to an unlimited extent, is the embodiment in accordance with the invention as defined in claim 2. For this solution, only two transistors with equal emitter areas are required, with currents of the same value then flowing through them.
In the normal case, all output transistors of the output current mirror will have equal transistor areas. The reference currents as defined in claim 3 are then to be dimensioned.
To compensate the base currents of the transistors of the output current mirror, the measures as defined in claim 4 are used in a further embodiment of the invention. It is achieved by means of these measures that the base currents are completely derived. This is achieved in that n+k compensation transistors for deriving the base currents are used in the case where the output current mirror has n output transistors and k input transistors. When m is the number of the second output transistors of the output current mirror, n+kxe2x88x92m base currents are drained to a power supply potential. To this end, n+kxe2x88x92m compensation transistors are provided. Furthermore, m base currents are drained by m compensation transistors, which currents are coupled to the input current of the second current mirror and are thus superimposed on the current supplied by the second output transistors of the output current mirror to this second current mirror. It is thereby achieved that the base current errors of the transistors of the second category of the output current mirror in the input current of the second current mirror are compensated.
A further embodiment of the invention as defined in claim 5 has the object and advantage that the circuit configuration of the transistors as defined in claim 5 ensures a safe start-up of the circuit arrangement when switching on the power supply. The additional current coupled into the first current mirror by this circuit arrangement serves for supplying a current from the output, ensuring flawless operation of the circuit arrangement when the power supply is switched on. However, the current is very small in proportion to the useful currents flowing in the output current mirror and in the first current mirror.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.